In recent years, MIMO (Multi-Input Multi-Output) transmission systems have been widely investigated to increase the channel capacity. The MIMO transmission may be categorized into two types, namely, a multi-stream transmission and a single-stream transmission. In the multi-stream transmission, the SDM (Space Division Multiplexing), as typified by V-BLAST (Vertical-Bell Laboratories Layered Space-Time), and an eigenmode transmission scheme that significantly increases the transmission channel capacity, have been proposed. In the single-stream transmission, the MRC (Maximal Ratio Combining) and the STBC (Space-Time Block Code) transmission, which may yield significant improvements in the SINR (Signal-to-Interference plus Noise Ratio) by space diversity, have been proposed.
To gain multiple-access capability in the MIMO transmission systems, the MIMO-CDMA (Code Division Multiple Access) has been studied. For downlink of the MIMO system, the STFS-CDMA (Space-Time-Frequency Spreading) has been proposed. The STFS-CDMA transmits a three-dimensional signal (space, time, and frequency) and employs the Walsh-Hadamard code, double orthogonal code, and the like. The STFS-CDMA employs CP (Cyclic Prefix) and MMSE (Minimum Mean-Squared Error) detector to improve its performance. The MIMO MC-CDMA systems that employ the STBC coding technique was proposed for the downlink MIMO transmission. However, these systems require a complete set of CSI (Channel-State Information), and thus, a complex computation process, in order to obtain a relatively high MIMO performance.